Fv-1n restriction in C3H and NIH/3T3 cell lines and DBA/2 secondary and tertiary mouse embryo cultures was studied using B-, and N- and NB-tropic murine leukemia viruses (MuLV) and pseudotypes of murine sarcoma viruses (MSV). Fv-1n restriction in DBA/2 cells was found to be similar to Fv-1b restriction previously reported in Balb/3T3 cells. The entire population of DBA/2 cells could be infected with restricted B-tropic virus at a high multiplicity of infection. C3H and NIH/3T3 cells gave somewhat different restriction patterns than did DBA/2 cells. It seems likely that in some Fv-1n cells, such as C3H and NIH/3T3, additional factors which inhibit MuLV replication can coexist with Fv-1. Mouse cell clones producing non-infections MuLV particles have been isolated in this laboratory; titration experiments have shown that these clones are infected with rescuable, replication-defective variants of MuLV.